Cross-site scripting (XSS) vulnerability in index.php in Simple Machines Forum (SMF) allows remote attackers to inject arbitrary web script or HTML via a base64 encoded params value in the action parameter.
Simple Machines Forum (SMF) 1-0-5 and earlier supports the use of URLs for avatar images, which allows remote attackers to monitor sensitive information of forum visitors such as IP address and user agent, as demonstrated using a PHP script on a malicious server.